1. Field of the Invention
The present invention relates to a sheet feeder for feeding sheets one by one in separation to an image forming unit of an image forming apparatus such as an electrophotographic copying machine, a laser printer or a facsimile apparatus from a sheet cassette.
2. Description of the Related Art
Heretofore, there have been developed various kinds of sheet feeders for feeding cut sheets of a predetermined size in an image forming apparatus such as a laser printer. A friction separation type sheet feeder is well known as one example, and will be explained with reference to FIG. 9.
In this friction separation type sheet feeder, numerous pieces of sheets P are stacked on a receiving plate 115, which is urged upward, housed inside a sheet cassette 102. A sheet feed shaft 117, which is intermittently rotated, is disposed above at the edge of the uppermost one P1 of the sheets P. In the middle of the feed shaft 117 is fixed a sheet feed roller 106 constituted by a member formed into a substantially D-shaped cross section and having a high friction coefficient at the circumference thereof. A pair of cylindrical sheet feed collars 116 each having a radius a little smaller than that of the circumference of the feed roller 106 are attached to both ends of the feed roller 106 for free rotation on the feed shaft 117. When the feed shaft 117 is rotated by a predetermined angle by a motor, the circumference of the feed roller 106 is pressed in contact with the uppermost sheet P1, which is pushed out forward (downstream of a feed direction) by friction generated on the circumference of the feed roller 106.
Sheets under the uppermost sheet P1 may be accidentally drawn out together. This phenomenon is generally called "dual feed." A separation pad holder 119 made of a synthetic resin is conventionally disposed near the uppermost sheet P1 just downstream of the feed direction for the purpose of prevention of dual feed. A separation pad 126 such as rubber having a friction coefficient smaller than that of a material of the feed roller 106 is attached onto the upper surface of the pad holder 119 (i.e., on a side facing the circumference of the feed roller 106). The pad holder 119 is urged so as to approach the circumference of the feed roller 106 by a spring 125.
In such a state that the circumference of the rotating feed roller 106 faces the pad 126, the pad holder 119 is pushed down against force of the spring 125 via the sheet. In this state, the uppermost sheet P1 is pushed forward by the friction force generated on the circumference of the feed roller 106. Because resistance for inhibiting pushing-out acts on the sheets under the uppermost sheet P1 by the friction force of the pad 126, only one sheet, i.e., only the uppermost sheet P1 can be pushed out to be fed between a pair of feed rollers 107 disposed downstream of the feed direction beyond the pad holder 119.
Meanwhile, in such a state that a flat portion 106a having a smaller radius faces the pad holder 119 after the feed roller 106 is rotated by a preset angle, the circumferences of the freely rotatable feed collars 116 each having a radius larger than that of the flat portion 106a are brought into contact with the upper surface of the sheet to be fed so that the reverse of the sheet comes into contact with the pad 126.
However, friction is generated between the reverse of the sheet and the surface of the pad 126 since the uppermost sheet P1 is pressed in contact with the feed collars 116 and the pad 126 while the sheet (the uppermost sheet P1), which has been fed in separation by the feed roller 106 and the pad 126, is fed by the pair of feed rollers 107, with an attendant problem of generation of large friction noise.